This invention relates to a device for transmitting, preferably via an optical fibre, measurement values of at least one sensor, which device comprises a transmitter circuit and a receiver circuit which includes an evaluation circuit.
A device of this kind transmits signals generated by a sensor from a transmitter circuit to a receiver circuit via a transmission line. For example, such a transmission line may be a coaxial cable. Preferably, the transmission line consists of an optical fibre. The electric signals generated by the sensor are converted into optical signals by a light transmitter, which optical signals are coupled into the optical fibre. The receiver circuit comprises a light receiver which converts the optical signals into electric signals again for supply to an evaluation unit. As a result of the transmission via optical fibres, the transmitted signal will not be influenced by electromagnetic interference fields.
A device of the kind set forth is known from EP-A 0 053 790, which corresponds to U.S. Pat. No. 4,346,478. The known device comprises a plurality of transmitter circuit with sensors which are capable of measuring, for example, a pressure or a temperature, by means of capacitive or resistive elements. The transmitter circuits are connected to a receiver circuit via optical fibres. Prior to the start of a measurement, the receiver circuit outputs a charge pulse whereby each time a capacitor in the transmitter circuits is charged. The energy taken up by the capacitor serves to power the other elements in a transmitter circuit during a measurement. After termination of the charge pulse, the receiver circuit outputs brief pulses in a given sequence, which pulses represent an address that are evaluated in the relevant transmitter circuits. After the subsequent appearance of a start pulse, the selected transmitter circuit starts the measurement. The transmitter circuit outputs an optical pulse whose start depends on the measurement result. It is also possible to generate a pulse where the measurement result depends on the width of the pulse. Because the transmitter circuits operate in a completely potential-free manner, the coupling in of sensor interference, for example, via an electric power supply system, is precluded. As a result of the fact that the operation is potential-free, the sensors can also be used in spaces where there is a risk of explosions. The sensors are included each time in an integrator circuit (RC member), the sensor being either a variable resistance element or a variable capacitive element. Due to this construction, use cannot be made of a four-pole sensor, for example a strain-gauge measuring bridge.
The described transmitter circuit has a complex construction because address evaluation takes place prior to each measurement. Moreover, continuous and fast transmission of measurement values is not possible because the capacitor for the energy supply in the transmitter circuit must be charged first, after which an address is evaluated and subsequently the measurement result is supplied after an integration.